High speed battery chargers, which charge a battery at a rapid rate, involve high energies, both for the charging pulses and the depolarization (discharging) pulses. U.S. Pat. Nos. 4,829,225 and 5,307,000 disclose a method and an apparatus for such high speed, high energy battery chargers. However, because of the high energies involved, catastrophic component failure in the charger, catastrophic failure of the battery being charged, and other problems can occur unless critical aspects of both the charge and depolarization waveforms are strictly controlled.
One problem occurs from the combination of the characteristic initial high current consumption and initial high current output of capacitive filters which use large capacitance values, such as the large capacitance values which are commonly present in switched-mode power supplies (SMPS), the inherent inductance of the interconnecting cables between the charger and the battery, and the initial charge acceptance profile of lead-acid batteries. This combination often results in very high magnitude current and voltage transients at the start of each charge pulse applied to the battery. These high magnitude leading edge transients, and their inherently high dv/dt and di/dt characteristics, can cause catastrophic electrical overstress failure of the circuit elements of the charging source, as well as cause high radiated and conducted EMI (electromagnetic interference) emissions. The high energy voltage reflections that propagate along battery cables due to high impulse pulse currents can easily exceed kV and cause immediate or latent failures to key circuit elements.
Induced voltages can be estimated as a function of E=L di/dt. Assuming a 350 nsec rise and fall times of the SMPS IGBT and an inductance of 206.2 nH per foot (676 nH per meter) for the battery cables, an 85A fundamental current surge will produce a 500 v reflection along a 10 foot (3.05 meter) taped-conductor cable. A 200 A current pulse will produce a 1176 v surge along that same cable. This analysis assumes that the conductors of the battery cable are in tight parallel proximity (taped together). If these same conductors are separated, the inductance is higher, and the reflected surge is higher. For example, if the conductors of the same battery cable are separated by only 12 inches (0.3 meter), the inductance will almost double, so the 85A current pulse will now produce a 910 volt surge along that separated battery cable, and the 200 A current pulse will produce a 2142 volt pulse along that separated battery cable.
Another problem occurs because of the high peak and continuous output charge current capabilities of lead-acid battery chargers. This high current output capability can result in a substantial overcurrent condition. Such an overcurrent condition can be caused by a hardware or software malfunction in the charger, such as in the controlling microprocessor, which malfunction manifests itself by excessive charger duty cycle demands.
Another type of problem occurs due to the depolarization pulse. The depolarization (discharging) circuit is placed directly across the battery terminals. Typically, an insulated gate bipolar transistor (IGBT) is used because of its ability to safely conduct a large current for a short period of time. However, a high capacity battery, such as a lead-acid battery, may be able to supply a large current for a period longer than the IGBT can tolerate. Thus, if a malfunction occurs that results in an excessively long depolarization pulse, or continuous depolarization signal, then the IGBT will overheat and fail. As the depolarization pulse is essential for the rapid, safe charging of batteries, the failure of the depolarization circuit will cause the battery to be improperly charged, and the battery may consequently be damaged or destroyed.
Still another type of problem occurs from the physical installation and location of the battery and of the charger. In order to accommodate the location of the lead-acid battery pack, it is often necessary to employ relatively long charge cables between the charger and the battery pack. These long cables have a substantial inductance and capacitance, and may have a length which is a substantial portion of a wavelength at the frequencies associated with the leading and trailing edge transients of the charge pulse and depolarization pulse waveforms. As a consequence, these long cables may act as a conductor which has a characteristic impedance. If that characteristic impedance is different than the impedance of the terminating load, that is, the battery, and that is almost always the case, then the change in impedance between the cable and the battery will result in a reflected waveform which can result in damaging overcurrents and overvoltages.
A typical battery charger will have a controller and a switching element, and the controller causes the switching element to connect the battery directly to a power source for the duration of a charge pulse. This causes a large current with a rapid rising edge to be applied to the battery. Likewise, when the charge pulse is completed, the controller causes the switching element to abruptly disconnect the battery from the power source. This causes a rapid falling edge. These rapid edges and large currents cause the problems described above.